The subject invention relates to thermoplastic polycarbonate/polyester compositions having low heat distortion temperatures which are characterized by high tensile and flexural strength and good impact strength. More particularly, the subject invention relates to thermoplastic compositions comprising an aromatic carbonate polymer and a polyester copolymer derived from a glycol portion comprising 1,4-cyclohexanedimethanol and ethylene glycol present in molar ratios of 1:4 to 4:1 respectively and an acid portion comprising at least one aromatic dicarboxylic acid selected from terephthalic acid, isophthalic acid and mixtures of the two.
Aromatic carbonate polymers are a well known and available family of materials which enjoy a variety of applications in the field of plastics. These polymers can be prepared by a number of procedures. In one way, the polymer is produced by reacting a dihydric phenol, e.g., 2,2-bis(4-hydroxyphenyl)propane, with a carbonate precursor, e.g., phosgene, in the presence of an acid binding agent.
In general, aromatic polycarbonate resins can be molded or otherwise shaped into articles which possess highly desirable chemical and physical properties, including resistance to attack by mineral acids, resistance to staining, high impact strength in thin walled sections and good dimensional stability. Polycarbonates are known to be admixable with various polyesters, including poly(alkylene terephthalates).
More particularly, it is known to blend polyesters with polycarbonates to provide thermoplastic compositions having improved properties over those based upon either of the single resins alone. Moreover, such blends are often more cost effective than polycarbonate alone.
Poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) have been widely used to form polyester-polycarbonate blends. For example, compositions of polycarbonate resins and polyester are disclosed by Nakamura et al. in U.S. Pat. No. 3,864,428.
Cohen et al., U.S. Pat. No. 4,257,937, describe thermoplastic molding compositions of poly(1,4-butylene terephthalate), optionally also containing poly(ethylene terephthalate), and a modifier composed of a combination of a polyacrylate resin and an aromatic polycarbonate. The compositions can also include fillers and/or reinforcements and flame retardant additives.
Fromuth et al., U.S. Pat. No. 4,264,487, disclose compositions based on aromatic polyesters such as polymeric resins of C.sub.2 to C.sub.8 alkylene terephthalates, which contain synergistic amounts of acrylate-based core-shell polymer and aromatic polycarbonate. The compositions are described as having high impact strength and an increased heat deflection temperature.
Still other modified polyester compositions are described elsewhere in the patent literature. Dieck et al. in U.S. Pat. No. 4,220,735 indicate that a polyblend composed of poly(1,4-butylene terephthalate) resin and poly(ethylene terephthalate) resin can be modified for greater impact strength by including effective amounts of a selectively hydrogenated monoalkenyl arene-diene block copolymer, for example, polystyrene-polybutadiene-polystyrene, together with an aromatic polycarbonate resin.
Gergen et al. in U.S. Pat. No. 4,111,895 disclose multi-component polymer blends comprised of polycarbonate, a selectively hydrogenated monoalkenyl arene-diene block copolymer and at least one dissimilar engineering thermoplastic, for example, thermoplastic polyesters. The components are admixed under conditions such that at least two of the polymers form continuous network phases which interlock with other polymer networks to produce a desirable balance of properties.
Bussink et al. in U.S. Pat. No. 4,267,096 teach that the use of a selectively hydrogenated elastomeric block copolymer together with an amorphous polyester improves the melt flow characteristics, resistance to brittle failure and resistance to environmental stress cracking of polycarbonate resin.
Japanese laid open patent application No. 044,373 describes thermoplastic molding compositions of polyester resin, e.g., poly(ethylene terephthalate), polycarbonate resin and minor amounts of a third resin obtained by polymerizing a shell comprising aromatic hydrocarbon and, optionally, methacrylate or similar monomer onto a rubbery acrylic core. The composition of these ingredients is said to have excellent moldability, mechanical and thermal properties.
A disadvantage associated with use of poly(alkylene terephthalate) is its relatively low notched impact strength, which this carries over into blends of the polyester with aromatic polycarbonates. It has been proposed that the notched impact strength of poly(alkylene terephthalates) can be improved upon by admixture with an impact modifier system composed of an aromatic polycarbonate and acrylate based polymer. Compositions of this type are disclosed in U.S. Pat. Nos. 4,257,937 (Cohen, et al.), 4,264,487 (Fromuth, et al.,) and the above-mentioned Japanese patent publication.
More recently, certain amorphous copolyesters, i.e., those having a low degree of crystallinity, have been developed. In U.S. Pat. No. 2,901,466 to Kibler et al., substantially amorphous polymeric linear polyesters and polyesteramides are described which are the condensation product of (1) either the cis or the trans isomer or a mixture of these isomers of 1,4-cyclohexanedimethanol alone or mixed with another bifunctional reactant, with (2) a bifunctional carboxy compound. The broad range of polymers defined therein include linear polyester polymers which have melting points as low as about 100.degree. C. and the polyesters melting below about 200.degree. C. are described as primarily useful for the preparation of molding compositions.
It has elsewhere more recently been disclosed that amorphous copolyesters having a low degree of crystallinity may be utilized in polycarbonate blends to provide improvements in impact strength and transparency, processability, solvent resistance and environmental stress cracking resistance.
More particularly, as disclosed in U.S. Pat. No. 4,188,314, assigned to the same assignee as the present invention, a composition useful for forming shaped articles characterized by enhanced solvent resistance and good physical properties comprises an aromatic carbonate polymer and a polyester polymer derived from cyclohexanedimethanol and a mixture of terephthalic acid and isophthalic acid. Articles shaped from these compositions are described as having higher dimensional stability than articles derived from cyclohexanedimethanol and a single acid, e.g., either terephthalic or isophthalic acid alone. In addition, the compositions are said to have lower vacuum forming temperatures thereby permitting sheets of the material to be vacuum formed or thermally shaped into shields or signs without the normally required time-consuming step of predrying the sheet. Finally, it is disclosed that the compositions can accept relatively larger amounts of UV stabilizers without the usual deleterious effect on impact strength which is associated with polycarbonate compositions containing similar concentrations of stabilizers which tend to degrade and become brittle.
Another thermoplastic resin blend incorporating a polycarbonate and an amorphous polyester is disclosed in U.S. Pat. No. 4,267,096, said patent also being assigned to the same assignee as the present invention. As described therein, useful compositions having improved processability and extrusion characteristics comprise: (A) from about 0.1 to about 6 parts by weight of a selectively hydrogenated linear, sequential or radial teleblock copolymer of a vinyl aromatic compound A and A' an olefinic elastomer (B) of the A--B--A'; A--(BAB).sub.n --A; A(BA).sub.n B; A.sub.4 B; A(B).sub.4 ; or B[(AB).sub.4 type, wherein n is an integer of from 1 to 10; (B) from about 65 to about 97.5 parts by weight of an aromatic polycarbonate resin; and (c) about 1 to about 30 parts by weight of an essentially amorphous polyester resin. The amorphous polyester resin component (c) is described as the reaction product of a C.sub.2 -C.sub.10 alkylene glycol and an aromatic dicarboxylic acid e.g., terephthalic or isophthalic acid. The use of copolyesters of poly(alkylene terephthalate) type is preferred. More particularly, it is preferred to use copolyesters of from 99.5% to 94% by weight of poly(alkylene terephthalate) which contain, incorporated at random in the chain small amounts of from 0.5 to 5% by weight of dissimilar units in order to break down any tendency whatever for the "100%" pure polyester to crystallize. Alternatively, the use of a small amount of isophthalic acid instead of terephthalic 100% will also produce satisfactory amorphous polyesters. The use of the above-described amorphous poly(alkylene terephthalates) in the selectively hydrogenated block copolymer/polycarbonate blends is said to provide compositions which extrude smoothly and are easy to strand, each without excessive die swell. Articles made from these blends may be used at temperatures far above the glass transition temperature of the crystalline polyesters without loss in important properties.
It has now been discovered that new and improved polycarbonate/polyester blend compositions may be prepared which incorporate still another amorphous polyester copolymer. The thermoplastic compositions of this invention form either a single or a two-phase system as evidenced by the fact that they exhibit either one or two glass transition temperatures. The compositions of the subject invention exhibit significantly lower heat distortion temperatures and in addition retain unexpectedly high tensile and flexural strengths and good impact strength over polycarbonate/polyethylene terephthalate blends.
In the past, articles molded from polycarbonates and poly(alkylene terephthalate)/polycarbonate resin blends had high heat distortion temperatures which required very high extrusion and molding temperatures in order to provide sufficient melt flow to the thermoplastic material to completely fill the interstices within molds. At these high molding and extrusion temperatures, degradation of the polymeric material frequently occurs which is often evidenced by discoloration of the material. The degradation of the material also contributes to a loss in impact strength, which is undesirable because high impact strength is one of the more functionally important attributes for shaped thermoplastic articles. The compositions of the present invention exhibit lower heat distortion temperatures, thereby permitting the compositions to be extruded and molded at lower processing temperature with significant reduction in degradation of the materials. Other important physical and mechanical properties of articles shaped from the instant compositions, such as tensile, flexural, and impact strengths are fully retained or improved upon with respect to the heat distortion temperature compositions of the prior art.